The electrical art is familiar with the practice of producing electrical resistance elements by applying a film of electrically conductive material over an insulating substrate. Among the electro-conductive film materials employed are various forms of carbon, various metals, and, more recently, oxides. While the electro-conductive films may be deposited on any compatible, electrically insulating substrate, a substantial line of commercial resistors have been developed in recent years wherein a tin oxide film doped with antimony oxide is deposited on a glass substrate. The characteristics of such resistors, their method of production, and the history of their development, are described in U.S. Pat. No. 3,437,974 granted Apr. 8, 1969 to J. Spiegler.
The resistance value for an electrical film element may be substantially enhanced by selectively removing a portion of the film material to convert the film into an elongated ribbon of material. To this end, it is common practice to deposit the electrically conducting film on a cylindrical substrate, such as a glass rod, and to spiral the element by cutting, or otherwise removing a thin line of film, to form a helical path therein. Originally, resistors were spiraled by a process of mechanical cutting or abrasion such as described for example in U.S. Pat. No. 1,859,112 issued May 17, 1932 to I. Silberstein. This technique is still widely used in the art, but has certain serious disadvantages. For example, the Spiegler patent, mentioned earlier, is addressed specifically to the problem of mechanical damage to the glass substrate from the cutting or abrading operation. The patent proposes a mechanically strong glass substrate which comprises a glass core with a glass sheath on the core, the coefficient of expansion of the sheath being lower than that of the core to strengthen the substrate.
In addition to the mechanical damage problem described by Spiegler, mechanical cutting or abrasion may produce a jagged edge which limits the electrical precision attainable. Also, there is a distinct physical limit to the width of the path that may be cut in a film, thereby limiting the number of spirals per inch that may be attained and, consequently, the value of electrical resistance attainable in this manner.